Since the Ziegler-Natta catalyst was developed in the middle of the 1950's, polyethylene, isotactic polypropylene and ethylene propylene diene copolymers (EPDM) of different properties have been produced. The Ziegler-Natta catalyst, of which the active sites are not uniform, is inapplicable to the preparation of polymers of narrow molecular weight distribution MwD and compositional distribution.
An appearance of a metallocene catalyst in the middle of the 1980's enabled to produce polymers of various properties that are otherwise impossible to produce with the Ziegler-Natta catalyst, as well as polymers of narrow molecular weight distribution and compositional distribution. In particular, the polymers of narrow molecular weight distribution and compositional distribution that are polymerized using the metallocene catalyst have a high strength relative to the polymers of broad molecular distribution and compositional distribution polymerized in the presence of the Ziegler-Natta catalyst, and reduce the stickiness in the manufacture of films or sheets. However, those polymers require an extremely high energy in processing because of its narrow molecular weight distribution.
Accordingly, many studies have been made in many countries of the world on the multinuclear metallocene catalysts for the sake of overcoming the drawbacks of the polymers of narrow molecular weight distribution. Examples of the related art can be described as follows.
First, U.S. Pat. No. 5,525,678 (Jun. 11, 1996) discloses a binuclear catalyst system for olefin polymerization that comprises a metallocene compound and a non-metallocene compound supported on a support, making it possible to, produce a high molecular weight polymer and a low molecular weight polymer at once. But, the catalyst system requires complex processes in regard to independent supporting of the metallocene compound and the non-metallocene compound and pretreatment of the support with different compounds for the supporting reaction.
U.S. Pat. No. 5,700,886 (Dec. 23, 1997) describes a polymerization method using at least two metallocene compounds as catalysts in a single reactor to control the molecular weight distribution of the polymer. This method is problematic in that a high expense for catalysts and strict polymerization conditions are required in the polymerization reaction to produce polymers of a desired molecular weight distribution because of using two or more metallocene compounds of a complicated structure.
Alternatively, U.S. Pat. No. 5,753,577 (May 19, 1998) discloses a binuclear metallocene compound containing two metal atoms in the group IV of the periodic table as central metals, which have an oxidation number of +3 and are linked together by a direct chemical bond, the metallocene compound also having a chemical bond bridging between ligands bonded to the central metals. The use of the catalyst for polymerization, however, results in polymers having a low molecular weight.
U.S. Pat. No. 5,442,020 (Aug. 15, 1995) discloses an ethylene, propylene or ethylene-alpha-olefin polymerization method using a binuclear metallocene compound that is prepared by reacting a group IV metal compound with an alkylene/silylene-bridged cyclopentadienyl group. But, the preparation of the catalyst requires a complex process in regard to introduction of alkylene or silylene groups to the catalyst.
U.S. Pat. No. 5,627,117 (May 6, 1997) describes an ethylene, propylene or ethylene-alpha-olefin polymerization method using a multinuclear metallocene compound that is prepared by reacting a group IV to VIII metal compound with a cyclopentadienyl group linked to alkylene or silylene groups or divalent germanium (Ge) or tin (Sn). But, the catalyst has a low activity for high-temperature polymerization relative to the existing catalysts.
In addition, U.S. Pat. No. 6,010,974 (Jan. 4, 2000) discloses a styrene polymerization method using a binuclear metallocene compound that is prepared by reacting a group IV metal compound with an alkylene/silylene-bridged cyclopentadienyl group. The use of this catalyst is however limited to the preparation of styrene polymer or styrene copolymer.